This invention generally relates to proximity sensors. More specifically, this invention relates to such a sensor which relies on a current induced by capacitive coupling of an article to the sensor as the means for indicating the presence of an article.
Proximity or article presence sensor which depend upon a capacitive coupling effect between the sensor and the article are known in the art. A very small current is generated within the sensor as a result of this coupling. To the best of my knowledge, the sensors of the prior art have attempted to use this current, or an associated voltage drop, directly as the primary signal. This has created operating problems since circuits operating with very small currents tend to be unstable and very prone to stray electrical noise distortion. In addition, an absolute level must be established in the circuit to allow determining when the induced current is great enough to signal the presence of an article. I have overcome these problems by measuring the current rise above a variable base line as the primary signal. The minimum circuit level with no article present is measured and held. The voltage and current resulting from the presence of an article are amplified. The negative and positive peak values from the amplifier are summed. This summed value is compared with the held minimum level and an output signal is generated if the two compared values differ by more than a pre-selected amount.